1. Field of the Invention
The present invention relates to a base band signal processing device for a radio transmitter to be used for a mobile communication base station and the like, and more particularly to a base band signal processing device for CDMA base station requiring handling of a signal that conforms to normal distribution having a big peak factor.
2. Description of Related Art
In recent years, CDMA having high utilization efficiency of frequency resources as a mobile communication system, and capable of broad band/high multiplex communication has received attention. In the CDMA system, it is known that since base band signals of more than one channel are spread into non-correlated signals in an artificial way by means of spreading codes that intersect each other at right angles and are transmitted with their codes multiplexed, when a number of multiplexing is increased, a transmission I, Q signal approaches the normal distribution. A signal presenting the normal property generates larger instantaneous peak amplitude than 10 dB on mean transmission power although low in the occurrence probability. A ratio of the instantaneous maximum power to the average power of such a signal is generally referred to peak factor.
If when transmitting a normal property signal by a radio transmitter, sufficient linearity is not secured even for large instantaneous peak amplitude, non-linear distortion occurs outside the transmission frequency band to become a disturbing wave for other systems. Its amount of occurrence has been severely regulated by the radio wave laws and regulations.
Under such circumstances, the radio transmitter, a power amplifier at the final stage, in particular, is compelled to be operated with the mean transmission power sufficiently lowered from the saturated output power, and the power efficiency becomes incapable of being sufficiently raised, as a result, leading to a problem that the device size and the running cost will be increased.
In order to solve such a problem, there has been devised the technique referred to as the so-called distortion compensation, in various ways for linearizing the power amplifier to a high degree to enable large output operation, and on the other hand, there also exists a method for enabling a large output operation of the amplifier by changing the distribution form itself of the base band signal to suppress the amount of occurrence of peak amplitude.
In the case of the latter technique, the signal quality is intrinsically deteriorated, but since the frequency of occurrence of the peak amplitude is sufficiently low in terms of the probability, this has little effect on the signal quality deterioration, and deterioration within a standard established for the applied system will be accepted.
As a simplest example, there is conceived a method for cutting off peak amplitude through the use of a limiter circuit, but since there is produced a break point that is not smooth in the signal in this case, the spectrum will be spread. As another method, there is also conceivable a method for band-limiting the limiter circuit output through the use of a filter, but the peak amplitude is reproduced by an operation of convolution due to the filter. As a conventional example of the technique for solving such a problem, there is named the system described in the Patent Literature 1.
First, with reference to FIG. 10, the description will be made of the conventional technique. FIG. 11 shows an example of operating waveform of the conventional technique. From a white normal property signal inputted, first a large amplitude component is cut off by a limiter 1001. In the case where this is band-limited by a filter 1006, when smoothing by the filter 1006, there may be cases where a peak amplitude more than the amplitude cut off by the limiter 1001 is reproduced. This is due to the operation of convolution in the filter 1006. Thus, a filter 1002 having the same as or similar characteristic to the filter 1006 will be used as a reference filter, and this output signal will be supplied to an amplitude control unit 1004. When an output signal from the amplitude control unit 1004 detects a value higher than the set value of the amplitude control unit 1004 concerning peak amplitude reproduced by the reference filter 1002, the output value will be lowered by a time period in which the convolution occurs at the filter 1006 at the later stage, that is, a period corresponding to the tap length of the filter 1006. On the other hand, a delay circuit 1003 which opposes corrects the signal delay that occurs in the reference filter 1002. Since the gain of the signal delayed is controlled by a multiplier 1005 on the basis of the output from the amplitude control unit 1004, the output value of the amplitude control unit is set appropriately, whereby it is possible to prevent a peak amplitude to be reproduced by the filter 1006 from exceeding the threshold.
As described above, in the conventional technique, by operations at two stages: cutting off the large amplitude by the limiter 1001 and lowering the gain by the amplitude control circuit 1004, the peak amplitude is suppressed. However, since in the latter operation, the gain is uniformly lowered by a time period corresponding to the tap length of the filter 1006 in order to avoid the influence of convolution although the actual time interval in that the peak amplitude occurs is exceedingly short, the influence on the deterioration of the signal quality will be great. Also, in the conventional technique, no consideration has been given to a multi-carrier signal consisting of a plurality of modulation signals and carrier waves.